Crossover from fragile liquids to strong liquids near the glass transition created by isotropic two-body short-range interactions

Abstract

The extensive molecular-dynamics simulations on binary mixtures A80B20 with a Stillinger–Weber potential are performed to obtain two types of glass-forming liquids near the glass transition, fragile and strong liquids. The simulations are done for different mass ratios Q(=mB/mA) under the same potential with mA being fixed, where mα denotes a mass of α particle. The simulation results for the self-diffusion coefficient D are then analyzed by two types of master curves recently proposed as D=d0x−1(1−x)2+ηexp[62x3+η(1−x)2+η] with η=4/3 for fragile liquids and 5/3 for strong liquids, where x is a reduced inverse temperature and d0 a positive constant. Then, it is shown that for Q<Qc the simulation results for the diffusion coefficient obey the fragile master curve with η=4/3, while for Q>Qc they obey the strong master curve with η=5/3, where Qc≃20. The structural relaxation time τα and the β-relaxation time τβ are also shown to obey the power laws recently proposed as τα∼D−(1+μ) and τβ∼D−(1−μ) in a supercooled region, respectively, while τα∼τβ∼D−2/3 in a liquid region, where μ=2/(3(η+2)). Here μ≃1/5 for fragile liquids and 2/11 for strong liquids. These situations are exactly the same as those in usual glass-forming liquids. Thus, it is emphasized that two types of glass-forming liquids can be simply created by simple short-range potentials.